Evaluation of an aspartame loading test for the detection of heterozygotes for classical phenylketonuria

Abstract

Classical phenylketonuria (PKU) is an inborn error of metabolism of autosomal recessive inheritance characterized by the accumulation of phenylalanine (Phe) in tissues due to Phe‐4‐hydroxylase deficiency. Several methods have been developed for the detection of PKU heterozygotes based on the determination of plasma Phe and tyrosine (Tyr) levels, on the analysis of the Phe/Tyr and Phe2/Tyr ratios and on the use of discriminant functions. The objective of the present study was to test the value of loading with aspartame (a sweetener consisting of Phe, aspartate and methanol) for the identification of PKU carriers. The study was conducted on 22 obligate heterozygotes and 27 controls. Two blood samples were collected (under fasting conditions and 30 min after the loading) for fluorometric determination of Phe and Tyr. Phe, Phe/Tyr and Phe2/Tyr values were higher in heterozygotes, whereas Tyr was higher in controls in both situations investigated. Linear discriminant function was considered to be the best parameter for differentiation of the individuals in the two groups. Under the conditions employed in the present study, aspartame loading did not show any advantages in discriminating between PKU carriers and normal individuals when compared to the same analysis performed under fasting conditions.

Phenylalanine and Tyrosine Metabolism Analysis in Heterozygotes for Phenylketonuria and in Healthy Individuals

“…The variables Phe/Tyr and Phe 2 /Tyr were increased in the heterozygotes throughout the measured period. This finding is in agreement with those from Silva et al, 19 Song et al, 28 and Abdalla et al 29…”

“…The variables Phe/Tyr and Phe 2 /Tyr were increased in the heterozygotes throughout the measured period. This finding is in agreement with those from Silva et al, 19 Song et al, 28 and Abdalla et al 29 The analysis of the ROC curve revealed that the best parameters to differentiate PKU heterozygotes from healthy individuals were, in the order, Phe after 45 and 90 minutes of overload and Phe 2 /Tyr after 90 minutes. Of note, Phe 2 /Tyr at T90 had presented the best performance in the previous study from Abdalla et al 29 We intended to increase the differentiation degree between both groups.…”

“…The positive predictive value of the test was 94.4%, and the negative predictive value was 81.25%. In the study of Silva and colleageues, 19 the discriminant function that considered fasting condition was found to be more efficient than the one that considered the variables after aspartame overload. In that work, the discriminant function correctly classified 98% of individuals with 97% of accuracy.…”

“…In the control group, 3 falsepositive cases were detected (11.11%, 2 male and 1 female). In the work from Silva et al, 19 which used aspartame overload instead (100 mg/kg, containing 56 mg/kg of Phe), 3 participants (all women) from the control group presented with Phe values above the expected for healthy individuals and 1 PKU heterozygote displayed lower probable values for this condition. It should be highlighted that the exclusion criteria were previously determined in the present study.…”

“…10,11,[14][15][16][17][18] The use of such methods is simple and low cost and can be justified by the high frequency of heterozygotes in the population (1:50-1:70), as well as by its preventive potential, since it may allow genetic counseling and prenatal diagnosis. 19 …”